1. Field of the Invention
The present invention generally relates to integrated circuit (IC) devices and, more particularly, to trimming circuits used to adjust the levels of voltages generated internally to such devices.
2. Description of the Related Art
Integrated circuit (IC) devices often operate using various internally generated voltages in an effort to reduce sensitivity to fluctuating external voltage supplies. Internal voltage generating circuits used in such devices often include trimming circuits to adjust the generated voltages, for example, to compensate for variations introduced by the manufacturing process. Trimming circuits are adjusted to bring internally generated voltages as close as possible to a target voltage during a testing procedure. Typically, the trimming circuits are adjusted, via a set of one or more switches that may be open or closed to increase or decrease the level of the generated voltage.
For example, FIG. 1 illustrates a simplified example of a conventional trimming circuit 150 including a set of transfer switches 1520 and 1521 used to adjust (i.e., trim) the output voltage VOUT of a voltage generator circuit 144. As illustrated, VOUT may be generated as a function of a reference voltage VREF (supplied by a reference voltage generator 142) and a voltage divider circuit (made up of resistors R0, R1, RA and RB). As VOUT may be supplied to a large number of components distributed throughout a chip (e.g., via a power bus), VREF may be supplied, via a comparator 145 to a p-mos driver 147 that typically has a larger driving capability than the reference voltage generator 142.
As illustrated, the set of transfer switches 1520 and 1521 may be used to vary VOUT by varying the effective resistance between nodes NB and N0 (hereinafter referred to as RTRIM) by selectively shunting across resistors RA and RB. VOUT may be expressed as a function of VREF and RTRIM according to the following formula:
      V    OUT    =            (              1        +                                            R              1                        +                          R              TRIM                                            R            0                              )        ·          V      REF      Assuming (for the moment) the transfer switches have no associated channel resistance, RTRIM may be varied from zero to RA+RB. The transfer switches 1520 and 1521 are typically controlled by a set of control signals C[0] and C[1] derived from a set of fuses 160 or bits in a register, either of which are typically set during a calibration or test procedure in which VOUT is compared to an externally supplied target voltage. For example, one or more of the fuses 160 may be cut (or blown) to increase or decrease the value of VOUT, in an effort to match the target voltage as closely as possible.
As illustrated, in a base setting, with both fuses intact (e.g., not blown or cut), C[0]=0 and C[1]=0, the transfer switch 1521 may be closed, while the transfer switch 1520 may be open, effectively removing RB from RTRIM (the path of the flow of current IBASE for the base setting is shown as a dotted line). Thus, the base setting for VOUT may be expressed, by modifying the equation above, as:
      V    OUT    =            (              1        +                                            R              1                        +                          R              A                                            R            0                              )        ·          V      REF      In order to decrease VOUT, the first fuse can be cut, as shown in the table 110 of FIG. 1, effectively removing RA from RTRIM, as well, making the numerator smaller. Thus, a decreased setting for VOUT may be expressed as:
      V    OUT    =            (              1        +                              R            1                                R            0                              )        ·                  V        REF            .      
Alternatively, the second fuse could be cut (C[1]=1), leaving the first fuse intact (C[0]=0), effectively adding RB to RTRIM. Thus, an increased setting for VOUT may be expressed as:
      V    OUT    =            (              1        +                                            R              1                        +                          R              A                        +                          R              B                                            R            0                              )        ·          V      REF      Cutting both fuses may increase or decrease VOUT, depending on the values of RA and RB. In other words, if RA>RB, cutting both fuses will decrease VOUT, if RA<RB, cutting both fuses will increase VOUT, relative to the base setting.
As previously stated, the equations listed above assume zero channel resistance of the transfer switches. However, this is not a valid assumption, as there is some finite channel resistance associated with each transfer switch (RXFER). Thus, to include the effect of series channel resistance of the transfer switch 1521, the equation for the base setting of VOUT should be modified as follows:
      V    OUT    =            (              1        +                                            R              1                        +                          R              A                        +                          R              XFER                                            R            0                              )        ·          V      REF      Unfortunately, the value of channel resistance is subject to process variations and has been observed to generally increase with temperature and vary with changing operating voltages. Moreover, in most voltage generators, a number of fuses and switches are utilized to achieve the desired resolution of adjustment, which will increase the effect of channel resistance. As an example, if a trim circuit has N transfer switches in series at any given time, the channel resistance will increase by a factor of N, such that:
      V    OUT    =            (              1        +                                            R              1                        +                          R              A                        +                                          R                XFER                            ·              N                                            R            0                              )        ·          V      REF      Thus, the level of VOUT may vary substantially with changing temperatures and operating voltages, particularly if the trimming circuit has a relatively large number of switches.
Accordingly, there is a need for an improved trimming circuit for use with a voltage generator, preferably with reduced sensitivity to transfer switch channel resistance.